The present inventive concept relates to semiconductor memory devices. More particularly, the inventive concept relates to nonvolatile semiconductor memory devices and methods of analyzing the threshold voltage state of constituent memory cells.
Semiconductor memory devices may be classified as volatile and nonvolatile. A volatile semiconductor device enjoys relatively fast data access speed, but loses stored data in the absence of applied power. In contrast, a nonvolatile semiconductor memory device retains stored data even in the absence of applied power. Accordingly, nonvolatile memory devices are commonly used to store critical data that must be retained regardless of the power supply state.
Among the broader class of nonvolatile memory devices, flash memory is able to electrically erase data stored in a defined block of memory cells. As a result, flash memory is used extensively in various computer memories and memory cards. Flash memory may be further classified as NOR type or NAND type according to the connection structure between the memory cell and bit line. In NOR flash memory, more than two cell transistors are connected in parallel to one bit line. Data is stored using a Channel Hot Electron (CHE) method and data are erased using a Fowler-Nordheim (F-N) tunneling method. In NAND flash memory, more than two cell transistors are connected in series to one bit line, and data is both stored and erased using the F-N tunneling method.
Each memory cell in a flash memory stores N-bit data, where “N” is a positive integer. In flash memory cells storing single bit (binary) data, two (2) threshold voltage states are used, respectively corresponding to data values of 1 and 0. In flash memory storing two-bit data, four (4) threshold voltage states are used, respectively corresponding to data values of 00, 01, 10, and 11. This approach extends to other types of multi-bit flash memory cells storing, for example, three-bit data according to eight (8) threshold voltage states, or four-bit data according to sixteen (16) threshold voltage states.